Related art and background information are discussed below in regard to membranes, membrane support fabrics, porogens, interpenetrating polymer networks, degradable fabrics, polysaccharides, and chromatography.
Thin materials of natural or synthetic composition which are permeable to fluids are commonly referred to as membranes. Typically, synthetic membranes are composite materials which are formed by casting, coating, or impregnating a polymeric solution onto a support substrate known as a membrane support fabric. In this manner, a polymer is applied to a supporting material, permeability is determined by intrinsic properties of the polymer and/or by use of porogens, and the resulting materials are used for filtration, chromatography, controlled drug release, etc.
Other applications of these materials are as integral components in devices for medical diagnostics, devices for medical therapeutics such as wound dressings, infection control, drug delivery, cell therapy, and in industrial applications related to food and beverage processing. Furthermore, there are applications in research products involved in the processing of samples for purposes of drug discovery and related laboratory operations for the purpose of generating information.
Numerous techniques are used for generation of the membrane pore structure. Common synthetic strategies rely on the use of porogens such as solvents or gases added to the polymer forming solution for the purpose of creating pores by physically or chemically induced phase separation and the microstructure of the resulting materials is generally characterized by a broad pore size distribution. In addition, approaches using porogen templates have been developed and such templates are able to induce specific structural pores within the residual structures. These templates can be small molecules, macromolecules, or polymeric structures depending on the desired pore structure. Another method for generating porous materials relies on the use of interpenetrating polymer networks (IPN) which are partially degradable and in which the pore structure is created by selective degradation of one of the polymers.
Membranes. Filtration membranes are highly efficient polymeric media for sub-micron separation tasks. Due to their fragile nature, the polymeric materials often need physical support for better handling or to withstand the operation conditions of the end use application. Accordingly, woven and nonwoven fabrics are commonly used for membrane casting, membrane lamination, and pleat support/drainage layers. In this manner, a membrane is prepared with characteristics suitable for microfiltration, ultrafiltration, or as a reverse osmosis membrane in flat sheet, spiral, or tubular filter configurations. An example is U.S. Pat. No. 7,316,919 which describes composite materials comprising supported macroporous gels and teaches that macroporous gels can be supported by a “support member” such that the macroporous gel fills the pores of the support laterally i.e. substantially perpendicular to the flow through the composite material and that the support member can be a fibrous nonwoven. The methods discussed in this patent for making these membranes involve complicated chemistry, hazardous solvents, and unreliable mechanical methods which result in low reproducibility and unpredictable permeability characteristics.
Membrane Support Fabrics. Membranes are constructed from highly engineered compounds using wet cast, polymeric materials such as poly(vinylidene difluoride), cellulose acetate, polyether sulfone, polyacrylamide, etc. During manufacture, the polymer compound is cast throughout or onto one side of a fabric support structure onto which a polymer is formed (E. Gregor, ‘Membrane Support Fabrics’, Water Conditioning & Purif. 50 (2008)). The thin membrane fabric substrate (or support) includes a nonwoven or woven, fabric. Its purpose is to provide a support material which can be fully or partially impregnated with a polymer forming solution as well as dimensional stability, strength, tear resistance, durability, and thereby allowing for processing into sheet-form or spiral wound modules.
The majority of membrane support fabrics utilize a polyester fabric as the base substrate, whereas polypropylene fabric substrates are used when chemical resistance is essential. The typical membrane support fabric is constructed as a wet-laid nonwoven fabric, whether polyester or polypropylene. Air-laid nonwovens, spunbond nonwovens, meltblown nonwovens and woven fabrics have been used as membrane support fabrics but wet-laid nonwovens are more uniform and consistent and are more commonly used for membrane manufacture. Examples are as follows:
U.S. Pat. Nos. 4,728,394 and 4,795,559 disclose a nonwoven support layer for casting semi-permeable membranes comprising a laminate of a low density layer made entirely from air-laid or carded polyester fibers containing 20 to 80% undrawn polyester or bicomponent polyester fibers, and a high density layer.
U.S. Pat. No. 5,989,432 discloses a composite membrane including a semi-permeable membrane, a support layer and a nonwoven web of multi-component fibers wherein the multi-component fibers comprise a first polymer as the core component and a second polymer on the surface of the fibers, the second polymer having a softening temperature below the softening temperatures of the first polymer, the membrane and the support layer.
U.S. Pat. No. 7,048,885 discloses an asymmetric nonwoven support layer having a microporous casting layer having a mean pore size no greater than about 300 micrometers on the surface thereof for casting semi-permeable membranes, the casting layer formed by heat treating, calendering, melt-blowing or wet-laying a layer of fibers.
U.S. Patent Application No. 2007/0138084 discloses a multilayer, membrane support fabric designed to permit a polymer forming solution to penetrate to a controlled depth without penetrating through the entire thickness of the fabric.
Porogens. Porosity is a measure of the void space in a material. Porosity is generally referred to as the void fraction or void volume and is typically reported as a value between 0-100%. For polymeric materials including hydrocolloids and hydrogels, porosity is a combination of chemical properties of the polymer and the properties of the polymerization process including but not limited to the properties and amounts of solvents and other additives used during polymerization. As a result, the overall porosity of a polymeric material is a combination of the intrinsic pore structure of the polymeric matrix and process related phenomena such as polymerization in the presence of inert components (solid, liquid or gas) commonly referred to as porogens, and phenomena such as solvent induced phase separation during polymerization, or processes that affect the polymer after formation such as mechanical fracturing or hole punching.
There are other methods for the manufacture of porous polymers involving the polymerization of a polymerizable component in the presence of an inert material referred to as a porogen. Subsequent leaching of insoluble porogens gives rise to interstices throughout the formed polymer material. However, these methods are usually complicated by extensive extraction procedures necessary for complete removal of the conventionally used porogens. A further disadvantage of these methods is the difficulty of stabilizing the suspension of porogen in the polymerization mixture. Another disadvantage of existing methods is that trace amounts of a miscible or immiscible porogen can affect the final or natural ultrastructure of the polymer in an unpredictable manner. Also, unstable suspensions can lead to a non-homogeneous and unacceptable product. In many cases, extensive optimization of the viscosity of the system and the type of porogen is needed to obtain a satisfactory and reproducible result. Also, existing procedures are limited in terms of the availability of porogens suitable for introducing the desired ranges of pore sizes. Furthermore, existing types of conventional, inert porogens lead to a broad pore size distribution and they require labor intensive and time consuming experimentation and optimization in order to obtain a useful material. Indeed, the pore size of polymers and hydrogels prepared by these porogen techniques depend on the size and/or concentration of the porogen but results are not predictable. Also, the introduction of a porogen reduces the mechanical strength significantly and the presence of large and variable sized pores will make the porous materials extremely weak.
Interpenetrating Polymer Networks (IPN). IPN's are generally defined (see IUPAC Compendium of Chemical Terminology, The Gold Book, 2nd Ed., A. D. McNaught and A. Wilkinson, Blackwell Science, 1977) as a combination of two independent polymers in a network form, at least one of which has been synthesized in the presence of the other. These complex polymer structures are of particular interest when they include two components with contrasted degradability under specific conditions. Accordingly, porous materials can be generated from such IPN's by resorting to selective degradation methods. However, by definition the polymer networks of an IPN are at least partially interlaced on a molecular scale and the IPN's reported in the literature are typically the result of intermixing two polymers on a molecular level. Hence, due to the normal molecular scale of the interlocking networks of interpenetrating polymer networks, macrophase segregation is prevented and the spatial scale of phase separation is restricted to hundredths or even tenths of nanometers. Thus, pore sizes created by this technique are restricted to hundredths or tenths of nanometers. In addition, IPN methods which rely on the intermixing of multiple reactants in a single phase result in changes to the intrinsic nature and pore structure of each polymeric component due to intermolecular interactions between the two or more partners and modified intramolecular interactions caused by the presence of one or the other polymer forming solutions. Examples are as follows:
U.S. Pat. No. 5,837,752 describes compositions for bone repair based on semi-interpenetrating polymer networks containing linear degradable polymers such as polyanhydrides and polyhydroxy acids.
U.S. Pat. No. 6,224,893 describes compositions for tissue engineering and drug delivery based on solutions of two or more polymers which form semi-interpenetrating or interpenetrating polymer networks.
U.S. Patent Application No. 2008/0237133 describes macroporous materials prepared from novel copolymers which can be made using an interpenetrating polymer network technique and having large pores typically in the range of 5,000 to 200,000 Angstroms. Preparations of porous materials from partially degradable interpenetrating polymer networks have been described in a number of publications. (Polymer Bulletin 61 (2008) 129-135, Polymer 48 (2007) 7017-7028, Macromolecules 38 (2005) 7274-7285, and Polymer News 29 (2004) 205-212). These publications describe the preparation of IPN's containing at least one degradable or dispersible polymer which, after removal by solvent or hydrolysis, results in a porous polymer product having typical pore sizes from 10-100 nanometers.
Degradable Fabrics. Fabrics and other materials composed of dispersible/degradable fibers are of particular interest for applications ranging from the common diaper to sophisticated medical applications such as scaffolds for in vitro and in vivo cell growth or organ implants. These materials typically utilize aliphatic polyester fibers which can be easily hydrolyzed and are based on alphahydroxyalkanoates such as polyglycolic acid (PGA) and polylactic acid (PLA). Also, other dispersible fiber materials such as polyvinylalcohol (PVA) and fiber based on lactides, lactones, carbonates and oxalates are commonly used when a dispersible or degradable fiber matrix is needed. Examples are as follows:
U.S. Pat. No. 4,633,873, and U.S. Pat. No. 4,871,365, describe a mesh or fabric made from adsorbable, partially adsorbable, or a mixture of adsorbable and non-adsorbable fibers and is a biodegradable fabric useful in surgical repair of soft tissue.
U.S. Pat. No. 5,092,884 describes a surgical composite structure with adsorbable and nonadsorbable components.
U.S. Pat. No. 5,437,918 describes a degradable nonwoven fabric and preparation process thereof.
U.S. Pat. No. 6,201,068 describes a biodegradable polylactide nonwoven material.
U.S. Pat. No. 7,265,188 relates to tough and ductile biodegradable, aliphatic polyester blend compositions and methods for preparing such compositions.
Polysaccharides.
U.S. Pat. No. 5,277,915 describes a gel-in-matrix composite including a three dimensional porous matrix, such as an open-cell foam, having within its matrix a mechanically-fractured hydrogel such as agarose and containing a network of fracture channels resultant from mechanically fracturing the gel.
U.S. Pat. No. 7,479,222 describes a porous adsorptive or chromatographic media comprised of a porous non-woven fabric with a coating of agarose that is about 1 micrometer to 40 micrometers thick.
U.S. Patent Application No. 2008/0154031 describes a method of preparing a separation matrix and specifically describes a method for preparing a particulate polysaccharide separation matrix such as agarose, which allows cross-linking with reduced risk of aggregation.
Chromatography. Chromatography is a general separation technique that uses the distribution of the molecules of interest between a functionalized stationary phase and a mobile phase for molecular separation. The stationary phase refers to a porous media and imbibed immobile solvent. Columns with associated end caps, fittings and tubing are the most common configuration, with the media packed into the tube or column. The mobile phase is pumped through the column. The sample is introduced at one end of the column, and the various components interact with the stationary phase and are adsorbed to or in the media or traverse the column at different velocities. The separated components are collected or detected at the other end of the column. Adsorbed components are released in a separate step by pumping an eluant solvent through the column. Chromatographic methods included among other methods, gel chromatography, ion exchange chromatography, hydrophobic interaction chromatography, reverse phase chromatography, affinity chromatography, immuno-adsorption chromatography, lectin affinity chromatography, ion affinity chromatography and other such well-known chromatographic methods. Current “state of the art” chromatographic or adsorptive separations use bead based, monolith or membrane media to accomplish the desired separation. These three technologies (beads, monoliths and membranes) accomplish separations via differing physical forms and therefore operate in phenomenologically different ways. A major difference between these three media is the relationship between the adsorbing surface (where adsorption of an entity to a ligand or ligands occurs) and the convective fluid flow.
Bead based media have convective flow occurring at the bead surface while most of the adsorbing surface is internal to the bead and can only be reached via diffusion. The convective fluid flow properties are determined by the bead size. Smaller beads require higher pressure to attain equivalent flow in a column. However, the equilibrium adsorbing capacity is not determined by the bead size. Therefore, the static capacity of the adsorbing surface and the flow properties of the materials are not necessarily coupled or interdependent. However, because most of the adsorbing capacity is accessed through diffusion, the dynamic binding capacity (capacity in a flow through mode at a given flow rate) is coupled to the bead size and therefore to the convective flow properties of the adsorbent. U.S. Pat. No. 6,428,707 describes adsorption/separation methods and a bead type media for adsorption/separation.
Typically in the area of chromatographic separations, polysaccharide polymers, such as agarose, are used to make gel media since agarose forms thermoreversible agarose gels in water at concentrations a low as 0.1%. To process agarose for example, dehydrated, amorphous agarose solid is suspended in water and heated to boiling at which point the agarose dissolves to form a homogeneous solution. The solution remains fluid and homogeneous as long as the temperature is above the gelling temperature, which is typically about 43° C. At and below the gelling temperature, a hydrogel is formed due to the formation of alpha-helical structures and other conformational changes in the agarose polymer matrix. The resultant gel takes on whatever shape the solution was just before gelling. Additionally, as the agarose approaches its gelling temperature, the viscosity of the solution becomes higher and higher as the hydrogel begins to form.
Traditionally, for the preparations of polysaccharide beads, such as those used in chromatography media, the heated solution is kept above its gel point and it is stirred into an immiscible, heated fluid, such as mineral or vegetable oil, to form beads in a well known process of suspension polymerization. The two phase material (beads of agarose in the immiscible fluid) is then cooled and the beads are recovered. The beads are diffusionally porous and can be used as made for size exclusion chromatography. Preferably, they are further processed by cross-linking, the addition of various capture chemistries such as affinity chemistries or ligands, positive or negative charge, hydrophobicity or combinations of cross-linking and chemistries to enhance their capture capabilities.
The beads are then loaded into a chromatography column forming a bed of media through which a fluid containing the material to be captured is passed. The beads are then washed to remove unbound contaminants and then the adsorbed, captured material is eluted from the beads and collected. Several problems exist with this type of media. The packing of the beads into a column is a difficult and laborious task. One needs to be sure that the column is properly packed so as to avoid channeling, bypass and blockages within the column. Packing of columns is time consuming and laborious. The use of beads limits the depth of the media in process applications because of the pressure that must be overcome. Excess pressure may compress the beads or require expensive pressure retaining components for the column. Softer beads tend to compress more than rigid beads. Compression is indicated by a steep increase in pressure drop across the bed at sufficiently high flow rates. High pressure drop is due to compression of the beads and subsequent reduction of void volume. The cumulative drag force of the flowing liquid through the bed causes higher pressure and results in compression of the beads. Drag force increases with higher flow rates, resulting in higher flow resistance and with bed height. One often needs to run a soft gel bead system at a slow rate in order to ensure that the pressure drop is within acceptable bounds and does not cause the beads to collapse and plug the column.
Other disadvantages to beaded supports for chromatography are the associated difficulties in manufacturing beads of uniform or similar size and composition. For example, agarose beads are commonly prepared by well known methods of suspension polymerization. In this process, a hot agarose solution is vigorously mixed with an inert, usually organic solvent in order to create a suspension of small particles. Then, after lowering the temperature of the mixture, the agarose particles gel and form mechanically stable beads. However, this commonly results in a distribution of bead sizes and subsequent mechanical separation techniques are employed in the manufacturing process in order to remove beads that are excessively large or small resulting in associated yield losses during the manufacturing process.
As the agarose beads are porous and the selected molecule to be captured must diffuse into the pores of the media to be adsorbed and captured, the speed and capacity of the system are diffusionally limited. There are two diffusional limitations, one surrounding the bead where a film of material may form and inhibit movement of the selected molecule to the surface of the bead and a second internal diffusional resistance which is determined by the size, number and length of the pores formed in the bead surface. Additionally, the permeability is related to bead size (which can vary widely) as well as the bead stability. Larger beads and beads with larger pores tend to have higher permeability. Beads that are not subject to or less subject to compression (by the weight of the beads above them coupled with the pressure under which the fluid flows through the bed) also tend to have greater permeability. However, at high flow rates, permeability decreases and dynamic capacity also decreases.
An alternative has been to use membrane or monolithic adsorbers which have a thin coating of a functional polymer such as agarose. One example of a surface functionalized monolith is taught by Cerro et al., Biotechnol. Prog 2003 (19) 921-927 (Use of ceramic monoliths as stationary phase in affinity chromatography), in which thin, surface-active only, agarose coatings on ceramic monoliths were created by impregnating the monolith with a hot solution of agarose, followed by removal of excess hot agarose solution from the cells within the monolith using compressed air and subsequently cooling the monolith to gel the agarose coating. One of the major problems with this coating process is that the coatings are difficult to effect on porous materials. In the article mentioned above, the agarose had to be applied in a heated state (thus requiring a substrate that is heat stable) making its application difficult to control as gelling occurred as the temperature dropped. A further problem is that only very thin coatings that have only surface activity can be created as occurs in membrane adsorbers. In part, this may be due to the method used for removing excess agarose. It may also be a function of the agarose gel point and the higher viscosity that occurs as the temperature of the agarose approaches the gel point. Moreover, the process would be very difficult if not impossible with substrates having pores that are relatively small in comparison to the cell size of the monoliths. The reason for these difficulties is that in some cases, air cannot be readily forced through certain porous materials without disrupting or otherwise damaging the porous structure as is the case with certain fabrics or porous structures.
U.S. Pat. No. 6,562,573 describes an approach for making an agarose coated substrate by methods that rely on forming a room temperature stable agarose solution through the use of high levels of chaotropic agents such as urea. These chaotropic agents create a modified, denatured form of agarose by interfering with stabilizing intra-molecular interactions mediated by non-covalent forces such as hydrogen bonds, van der Waals forces, and hydrophobic effects. A major problem with the method of U.S. Pat. No. 6,562,573 is that the process causes the pores of the substrate to be substantially blocked, severely limiting convective flow through the porous support. Additionally, the diffusional resistance is high, limiting the ability of the media to work rapidly.
U.S. Pat. No. 7,479,223 describes a porous adsorptive or chromatographic media comprised of a porous non-woven fabric with a coating of agarose that is about 1 micrometer to 40 micrometers thick. This method is similar to U.S. Pat. No. 6,562,573 in that it describes an approach for making an agarose coated substrate, by methods that rely on forming a room temperature stable agarose solution through the use of high levels of chaotropic agents such as urea. Water or ethanol or other reagents are carefully added to extract the chaotopic agent such that a gel-like precipitate forms at the interface between the agarose solution and the added reagent. This gel layer prevents migration of the agarose in the chaotropic mixture but allows further migration of the water, urea, or other chaotropic molecules out of the agarose solution and into the surrounding environment. Thus, the modified agarose solutions of U.S. Pat. No. 7,479,223 result in an agarose gel-like deposit that is in a chemically and physically altered state compared to native agarose gels prepared without additives. Again, as in U.S. Pat. No. 6,562,573, the process continues until the agarose solution turns into a “gel” within the interstices of the pores of a porous substrate or as a thin coating on the surface of the fibers of the substrate. Thus, a problem with this method is that only thin coatings that have only surface activity are created. In part, this may be due to the blotting or squeegee method used for removing excess agarose. Moreover, the blotting/squeegee process of this patent would be very difficult if not impossible with substrates having a pore structure that is relatively small. The reason for these difficulties is that in some cases, the squeegee process used in this method cannot readily force the coating solution through certain porous materials without disrupting or otherwise damaging the porous structure as is the case with certain fabrics or porous materials.
A schematic diagram of a conventional composite material 10 is shown in FIG. 1, according to U.S. Pat. No. 7,316,919 which describes a support member 12 comprising a plurality of (large) pores 16 and with a non self-supporting porous gel or polymer 14 being located in the spaces between the fibers of the support member. The porous pore structure of the composite material described in U.S. Pat. No. 7,316,919 is a random discontinuous pore structure due to modifications to the functional polymer by the use of porogens to form a heterogeneous polymer matrix with many holes or pores 16 in it. Thus, preparation of this composite material is a one step process using porogens to create significant chemical and physical modifications of the functional polymer leading to the final product.